Mirror assembly with flexible membrane

ABSTRACT

A flexible mirror (10) comprising a reflectively coated membrane (12) overlying a circular chamber (14) has its membrane (12) tension controlled around the membrane periphery. The membrane periphery is secured by circularly-extending clamp surfaces formed between two rings (21, 24) which are axially movably mounted by three bolts (22) and associated springs (23) which provide for coarse tension control when the membrane (12) is pulled down against the circular aperture (13) of structure (15). Fine tension control is provided by one ring (24) containing a segmented inner ring (32) the segments of which are independently operated by bolts (33) causing the segments to abut an O-ring (30) overlying membrane (12) and squashing the membrane (12) to a greater or less extent into an annular groove or channel (31) formed in the ring (21). The effectiveness of the tension control is monitored interferometrically.

This invention relates to mirror assemblies.

Mirrors are well known optical components having a large variety of usesand variable focal length mirrors have previously been proposedutilising a thin flexible carrier having a reflective surface, thecarrier with its reflective surface being displaced from planar bysuction. Particular forms of such mirror assemblies and which arecharacterised by good image forming characteristics are disclosed inEuropean Patent Publication No. 0152268.

It is an object of the present invention to provide an improved form ofmirror assembly.

According to the present invention there is provided a mirror assemblycomprising a rigid circular structure defining a hollow chamber having acircular aperture, a flexible membrane disposed across said circularaperture and in contact with the periphery of the circular aperture,said flexible membrane having a reflective coating adherent to themembrane face which is distal to said circular aperture,circularly-extending secural means peripherally engaging said membraneradially outwardly of said circular aperture and capable of renderingflat that portion of the membrane covering said circular aperture,pneumatic means connected to said chamber for varying the differentialpneumatic pressure across the membrane to thereby apply a pneumaticmembrane deformation force to the membrane, said reflective coatingbeing rendered variable in focal length according to the magnitude ofthe applied deformation force, said secural means providing peripherallycontinuous clamping of the membrane between opposed circularly-extendingclamp surfaces at least one of which is mounted for movement axially ofsaid circular structure in order to effect rendering said membraneportion flat in the absence of and deformation force applied by saidpneumatic means, said secural means including peripherally-discontinuousadjustment means for adjusting the tension in individual peripheralregions of said membrane portion, and stress control means located atthe interface between the circular structure and the reflective coatingfor controlling the stress distribution in the membrane around theperiphery of the circular aperture when deformation force is applied bysaid pneumatic means.

An embodiment of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of the embodiment; and

FIG. 2 is a plan view of a portion of the FIG. 1 embodiment.

In the drawings, mirror assembly 10 is formed by a reflective coating 11on one surface of a flexible substrate 12 which overlies a circularaperture 13 at the open end of a hollow chamber 14 formed by a circularstructure 15. The structure 15 comprises a radially thick ring wall 18which is secured by a plurality of bolts 16 to a rigid base plate 17 andprovision is made at the interface for a plastic O-ring to effect apneumatic seal. Base plate 17 is of greater diameter than the ring wall18 and the annular skirt thereby provided, supports acircularly-extending secural means 20 for peripherally engaging themembrane 12 radially outwardly of the circular aperture 13.

Secural means 20 comprises a lower ring element 21 which is resilientlysecured to base plate 17 by three equi-angularly spaced bolts 22 toprovide a three-point mounting, each bolt being surrounded by a spring23. With this arrangement adjustment of the plane of element 21 iseasily effected without introducing distortion of the ring element intoa non-planar condition. A second ring element 24 is secured to the firstring element 21 by means of a plurality of bolts 25. Bolts 25 may betwenty-four in number and the abutting surfaces of ring elements 21, 24,are substantially planar and provide peripherally continuous clamping ofthe membrane 12 between opposed circularly-extending clamp surfaces.Because of bolts 22 and springs 23 both of these surfaces are capable ofmovement axially with respect to the circular structure 15 for thepurpose of rendering the central portion of the membrane flat bytensioning it over the end face of structure 15 in the absence of anydeformation force applied to the membrane 12 by means of differentialpneumatic pressure. Variation in this differential pressure is effectedby applying suction to the interior of chamber 13 by means of a vacuumsource applied to nipple 26 mounted in base plate 17.

For the purpose of providing selective localised tension control inmembrane 12 around the circular periphery of the membrane 12 the securalmeans 20 incorporates an O-ring 30 which overlies membrane 12 and forcesthe membranes at least partly into an annular groove 31 formed in ringelement 21. O-ring 30 is forced into groove 31 by a segmented ring 32abutting bolts 33, both of which are mounted in ring 24. It is preferredthat ring 32 has twelve segments each of which is associated with twobolts 33 and it will be appreciated that by varying the penetration ofeach pair of bolts 33 into ring 24 the associated segment of ring 32 isforced against O-ring 30 to squash O-ring 30 to a greater or lesserextent into groove 31 and this provides for localised tension control ofthe membrane 12.

For the purpose of providing membrane stress control at the interfacebetween the circular structure 13 and the reflective coating 11 in thisconstruction the circular ring wall 18 is both radially broad and hasits end face 18A profiled into a slightly convex form in order toprovide a radially smooth contour which is circularly symmetric for themembrane 12. The radially broad nature of the wall 18 prevents vacuumleaks, the mirror can be held at any one curvature to interferometricaccuracy (as proved by interferometric testing), no continuously runningvacuum pump is required and the forces between wall 18 and membrane 12are always small. If ring wall 18 is not radially broad the forcesimposed by it on membrane 12 becomes large and membrane 12 experiencesvery large forces which will permanently crease the membrane 12 andcause large localised non-symmetric stretch therein, both of whichresult in non-symmetrical mirrors giving rise to poor imaging or noimaging at all.

By way of example it is preferred that ring wall 18 have a radialthickness of about 2 inches (50 mm) with a convex profile of maximumamplitude in the range 1/16th to 1/8th inches (2-4 mm) and that thediameter of the chamber 13 be about 8 inches (200 mm) and that themembrane 12, together with its reflective coating 11, has a thickness of125 microns. With this arrangement the contact radial width between themembrane and the ring wall is very many times greater than the thicknessof the membrane being, in this example, in the same order of magnitudeas the radius of the chamber 14, and is sufficiently large as tomaintain stress concentrations in the membrane 12, when the latter isdeformed to its maximum extent by pneumatic pressure, below the criticalyielding-stress level for the membrane material, being the stress levelat which the material yields inelastically. With this arrangement ofradially broad ringwall end face there is the additional advantage thatsubstantial membrane vibration damping is provided.

It will be appreciated that when suction is applied through nipple 26 tothe interior of chamber 13, membrane 12 is caused to deform from itsinitial extremely planar and uniformly tensioned condition andreflective surface 11 is formed into a concave mirror. The contact areaof membrane 12 on ring wall 18 is very substantial which controls thestresses in the membrane during use of the mirror so that for asubstantial range in f-number values of the reflective coating 11, downto F 0.5, the mirror has extremely good image forming qualities whichare at least as good as can be achieved by conventional ground glassmirrors. The image forming qualities of the mirrors are dependent tosome extent in the ability of the structure to produce a freedom fromstress variation around the membrane 12 in the presence and in theabsence of a deformation force applied to the membrane and this isfacilitated by the tension control arrangement 30-33.

Monitoring of the membrane 12, in the presence of a deformation force,and adjustment of the tension control arrangement 30-33 to minimise andpreferably eliminate stress variations may conveniently be undertaken byinterferometry. For example by directing an expanded beam of laserradiation onto the membrane 12 and causing the reflected beam to becollimated and to impinge upon the front and rear surfaces of anoptically flat surfaced plane-parallel sheet of transparent glass,interfering wave-fronts are established which form an interferencefringe pattern. For a perfectly tensioned membrane 12 and a perfectlymade sheet of glass this fringe pattern is then a series of parallellines of uniform width. Adjustment of bolts 33 enables distorted fringelines to be rendered straight and parallel and of uniform width bysqueezing O-ring 30 at desired localities around the periphery of themembrane 12 in a fine tension control mode. Adjustment of bolts 22provides coarse tension control over the entire surface area of themembrane and it is of course desirable that bolts 22 initially be set toprovide the best attainable coarse tension control prior to setting ofthe fine tension control arrangement 30-33.

I claim:
 1. A mirror assembly comprising a rigid circular structure (15)defining a hollow chamber (14) having a circular aperture (13), aflexible membrane (12) disposed across said circular aperture (13) andin contact with the periphery (18) of the circular aperture (13), saidflexible membrane (12) having a reflective coating (11) adherent to themembrane face which is distal to said circular aperture (13),circularly-extending secural means (20) peripherally engaging saidmembrane (12) radially outwardly of said circular aperture (13) andcapable of rendering flat that portion of the membrane (12) coveringsaid circular aperture (13), pneumatic means (26) connected to saidchamber (14) for varying the differential pneumatic pressure across themembrane (12) to thereby apply a pneumatic membrane deformation force tothe membrane (12), said reflective coating (11) being rendered variablein focal length according to the magnitude of the applied deformationforce, characterised in that said secural means (20) providesperipherally continuous clamping of the membrane (12) between opposedcircularly-extending clamp surfaces (21, 24) at least one of which ismounted for movement axially of said circular structure (15) in order toeffect rendering said membrane portion flat in the absence of anydeformation force applied by said pneumatic means (26), and in that saidsecural means (20) also provides peripherally-discontinuous adjustmentmeans (30-33) for adjusting tension in individual peripheral regions ofsaid membrane portion, and stress control means (18A) are providedlocated at the interface between the circular structure (15) and thereflective coating (11) for controlling the stress distribution in themembrane (12) around the periphery of the circular aperture (13) whendeformation force is applied by said pneumatic means (26).
 2. A mirrorassembly as claimed in claim 1, characterised in that the adjustmentmeans (30-33) comprises an O-ring (30) overlying the membrane (12) andco-operating with a circular channel (31) and a segmented ring (32)engaging the O-ring (30), the individual segments of said ring (32)being independently adjustable to engage the O-ring (30) which squashesthe membrane (12) into the channel (31).
 3. A mirror assembly as claimedin claim 2, characterised in that the adjustment means (30-33) iscarried by a ring (21) defining one of said axially movable clampsurfaces said ring (21) being resiliently mounted (22, 23) on saidstructure (15).
 4. A mirror assembly as claimed in claim 3,characterised in that the mounting (22, 23) for said ring (21) is athree-point mounting provided by bolts (22) equi-angularly spaced aroundthe periphery of the circular structure (15), springs (23) beingprovided between the ring (21) and the structure (15).
 5. A mirrorassembly as claimed in claim 2, characterised in that each ring segmentis associated with two bolts (33) for adjusting that segment relative tothe O-ring (30).